Current popular medical imaging techniques are X-ray imaging (including modalities such as Computerized Tomography and mammography), ultrasonic imaging and MRI (Magnetic Resonance Imaging). Since the 1980s, the use of microwave imaging has been discussed for mapping the interior of the human body and detecting anomalies such as malignant tumors.
Microwave imaging of the human body has developed significantly over the years. Breast imaging has been a popular potential application, both in view of its medical and social importance, and in view of the relatively low-loss materials of which a woman's breast is composed. Examples of such earlier works are U.S. Pat. No. 4,641,659 which utilizes a single scanning antenna, and U.S. Pat. No. 7,454,242, U.S. Pat. No. 6,421,550 and U.S. Pat. No. 7,164,105, which utilize arrays of antennas to replace the mechanical scanning and add bistatic measurements in addition to the monostatic reflection measurements.
All of the prior microwave-imaging attempts are hindered by the need to identify in-depth features in the human body through the outer attenuating body layers. The faint signal variations caused by in-depth features are masked by reflections from the antennas themselves and the tails of reflections from closer features, such as the interface with the skin. Substantial work has been done on calibrating the antenna arrays and cancelling out the contribution of the shallow layers so as not to disturb the detection of deeper features. Multiple algorithms were developed over time for reconstructing the spatial map of dielectric properties of the object from multi-antenna observations, starting with the simpler “delay-and-sum” (DAS) algorithms, continuing to intricate inverse-problem algorithms. Nevertheless, the methods suffer from residual errors and limited dynamic range.
Detection and characterization of blood flow in the body is another widely studied subject. Techniques based on ultrasonic Doppler detection of the flow of blood cells are in use. Impedance variations of the human body due to widening and narrowing of the blood vessels according to the cardiac rhythm are used to characterize hemodynamic parameters, for example in U.S. Pat. No. 5,469,859. Use of microwave Doppler detection to estimate hemodynamic parameters was proposed in U.S. Pat. No. 4,926,868. Detection of cardiac and respiratory functions was further discussed in U.S. Pat. No. 4,991,585. Another work on microwave detection of movement of internal organs, assisted by time gating, is described by McEwan in U.S. Pat. No. 5,573,012 and U.S. Pat. No. 5,766,208. None of these works combines these principles with multi-static MIMO radar measurements. Notably, McEwan's patents use single transmit-receive antenna pair and, in spite of using the term “imaging” in the patent's title, no formation of two- or three-dimensional image is performed.
US published patent application 2004/0015087 describes use of an antenna array for heart size measurement in which each of the antennas is used to obtain a Doppler signal. The signals are then used to form a two-dimensional image by associating the Doppler signal from each antenna with a pixel area in an image. This application does not describe overlaying of Doppler-induced image data with microwave imaging spatial data. Moreover, there is no reconstruction of a spatial image other than the crude representation of the Doppler data for the purpose of heart size measurement.